Recently, LEDs emitting light of short wavelength, such as a blue LED, an ultraviolet LED, etc., have been developed and provided for practical use. These LEDs are used in wide applications including, for example, in lighting and illumination, replacing the existing fluorescent lamps or incandescent lamps, and as light sources for curing UV-curable resins or UV-curable inks, replacing the existing short arc lamps.
In general, an LED has an LED die having an anode and a cathode formed on the surface thereof. The anode and the cathode are respectively wire-bonded to an external electrode and the LED die emits light as it is electrically connected to the external electrode.
If the LED die and the ultrafine wire (with a diameter of, e.g., 30 μm) of the LED are exposed to outside, the LED die may be damaged or the wire may be broken. For this reason, the LED is usually sealed with a sealer (e.g., a resin).
The sealing of the LED with a sealer is also effective in improving light extraction efficiency because, if the LED is sealed with a sealer with a higher refractive index than air, the difference in refractive indices at the interface of the LED die and the sealer is decreased.
As the sealer, an epoxy resin with high transparency has been used for LEDs emitting visible light (see, for example, the patent document 1). However, if the epoxy resin is used for LEDs emitting short-wavelength light, the resin itself may be deteriorated by the short-wavelength light, particularly the strong UV light emitted from the ultraviolet LED, which is used as a light source for curing UV-curable resins or UV-curable inks, resulting in coloration or cracking and thus decreased light emission.
For this reason, a silicone resin with superior heat resistance and light resistance has been proposed as the sealer (see, for example, the patent document 2).